On this date in 1866, noted British physician Sir Arthur Keith was born in Scotland. He became a well-known expert in anatomy and physical anthropology, and eventually identified (with Martin Flack) the sinoatrial node as the heart's natural pacemaker, now known as the sinoatrial node of Keith and Flack. This sinoatrial node, or sinus node, is a group of cells in the upper part of the heart's right atrium where electrical impulses originate. Once an impulse starts, it then travels cell by cell through the heart to the atrioventricular node, or AV. Like a gate, the AV curbs the electrical spark before it descends into the ventricles. This allows the atria to fully contract before the ventricles are stimulated. Keith is also known for his studies of fossilized human and humanoid remains and his research on tuberculosis. He died in 1954.

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